Greenhouse farming for cultivation of plants in a greenhouse is widely adopted because it brings much greater yield amount and much better quality than open field farming. Recently, for the purpose of further improvement of yield and quality, adjustment of picking season, reducing the period of cultivation, and so on, it has been attempted to convert ultraviolet light, which is harmful to plants, to blue light, which is helpful for photosynthesis, or to convert a light of green to yellow range, which has low photosynthesis efficiency, to a light of orange to red range, which has high efficiency of photosynthesis, by means of an agricultural film used for a greenhouse.
As a wavelength conversion film which has a function to convert a light with a specific wavelength to a light with a different wavelength (hereinafter referred to as wavelength converting function), the following one is proposed: an agricultural wavelength conversion material wherein a non-fluororesin film containing an organic ultraviolet absorbent and a non-fluororesin film containing two kinds of fluorescent colorants as wavelength converting components are laminated (Patent Document 1).
However, this wavelength conversion film has the following problems:
(1) the wavelength converting function decreases substantially after long-term use; and
(2) the visible light transmittance of the film itself is low; that is, the film itself absorbs light significantly and, as a result, the light intensity after conversion of the wavelength is lower than the light intensity with the wavelength in the sunlight before incidence.    Patent Document 1: JP-A-170865